Dynode coating



Aug. 6, 19,57

' D. DOBISCHEK E'EAL DYNODE COATING Filed Feb. 5, 1954 JNVENTQRS I DIETRICH DOBISCHEK 'JOHN B. FREELY HAROLD JACOBS United States Patent DYNODE'COATlNG Dietrich Dobisc'llek, Asb'ury 'Park, John B. Freely, :Red dtank, and Harold Jacobs, West Long Branch, N. J.,

assignors to the United Statesof Americans represented ;by-the Secretary'of the Army 'kppli'cation February '3, 1954, SeriaPNo'. 40sg'05a 1:1 Claims. cLcn-ms (Gram-saunter Title '35, U.s. Code 1952 sec.266)

flhere have already been many efforts to improve the emission characteristics of oxide coatings on emissive cathodes and on target electrodes which provide secondarytemission. As a result ofresearch carried out by the inventors, --it was first learned that if high D. C. fields are applied across certain oxide films, unusually lhigh secondary-emissionratios areproduced, the applied high D. C. field causing an enhancement of the secondary emission ratio. This enhanced emisison maybecome self-sustained if -a high enough D. C. is applied across thefilm.

It'is,--according1y, a primary object of thespresent invention -to;provide a method of :preparing a coating on an electrode in a discharge device which is stable, reproducibley-and capable of providing high, self-sustained secondary emission. I g

It is a further object to provide a discharge device wherein-such=a*coating-is adapted for use-such as a noisegener-a-tor. 1

-I t :is another object to provide a device wherein such a'coati-ng is adapted for use in a display tube.

. tn accordance with the present invention there is provided 'amethod Ofpreparingamagnesium oxide coat ihg for an-electrodein adischarge device, the coating heingcharacteriZetl byenhanced secondary emission when bombarded with electrons, comprising filling with-oxygen at a pressure of 20-40 mm. of mercury, anenvelope having-therein "a metal receptacle containing magnesium and an electrode spaced from said receptacle, initiating the positive ionization of's'aid oxygen,,positively biasing said electrode 'with respect to said-receptacle whereby a glow "discharge is *set up therebetweem'the positive ions being attracted ito'said receptacle thereby :igniting said magnesiu'm, permitting' saidignited magnesium to burn in said oxygen for "a predetermined ,period whereby a thin, sta ladtite like, ipor'ous magnesium oxide coating-is deposited onsaid 1 electrode.

Also in accordance with :the present invention there is provided a discharge device comprising an evacuated envelope includin'g therewithin atle'ast an emitting cathode and an anode having a stalactite like porous magne- 'sium c'oating thereon, saidicoating having a high secondary to primaryemission-ratio.

For a better understanding of the invention together with othe'r and further objects thereof, "reference is @had 'to the following description taken in connection-with 'the accompanying drawings and its scope will be pointed out'n the-appen'ded claims.

In -th'e drawing, 'Fig. 1 is *a;perspective"view partially cut away, o'f a discharge :device in accordance with a preferred embodiment of the present invention; Fig. 2 is a perspective view of a second embodiment; Fig. 3 is a 2,802,127 Patented Aug. 6, 1957 section taken along line 3-*-3 of Fig. 2 looking in the directionof the arrowsyFig. 4 is a perspective view of a third embodiment of the present invention and Fig. 5 is a section talcen along line 5-5 of Fig. 4 looking in the direction'of the arrows. 7

Referring more rparticularly to Fig. lthere is shown a device forvproviding the magnesium oxide coating of the present invention. evacuable tube 10 or chamber which is provided with means Knot shown) for introducing oxygen or other gas therein and with means (not shown), for exhausting gas therefrom has positioned therewithin an assembly comprising a metal receptacle 12 containing metal magnesium *foil. Opposing and spacedfrom receptacle .12 a predetermined distance is a first portion ot the surface of a rotatable cylindrical dynode 14. For rotatably positioningdynode 14 there ispr'ovided a tab -16 responsive to a magnetic field (not shown) exter-nal to tube 10, Spacedfrom the dynode is an accurately contoured :grid structure 18 concentric to a second portion of the'surface-of the dynode. Opposing and spaced from grid structure 13 is a cathode 20 which may be afilament 017a cathode of the indirectly heated type and whose emitting surface is 'in the same horizontalvplane as:receptacle 12. D. C. potential source 22 ,provides the necessary -eelctrode biasing potentials in the fabrication of the tube as will be hereinafter explained. --All the structural elements inside chamber 1i) are supported bygdevices well known in the art and itis, therefore, believed unnecessary for them @to be vset forth.

:In :operation, tube 10 -is filled withoxygen to a pressure of 29-40 mm. of :mercury. -The ionization of the oxygenis initiated tby-a spark from -a spark coil, by light, or other suitable means. Dynode 14:isthen made several hundred volts -more positive than :magnesium receptacle 12 so that -a glow discharge is initiated therebetween. A currentilimiting resistormay be connected in'the dynode receptacle circuit to prevent excess current from being drawn. The glow discharge results in a positive ion bombardment whichcauses the magnesium receptacle 1-2 to heat very rapidly, thereby igniting the -magnesium therein. l he :rnagnesium burns violently and deposits as -a nine, white stalactitetlike dayer magnesium 'oxide layer 30 on a spot ondynode 14. After the magnesium oxide layer 30-isso formed, the appliedpositive potential is disconnected from dynode 14, and the dynode is inductionvheatedat about 700 800 C. in the oxygen for about 30 .seconds to :complete the oxidation. After the latter oxidation step, theroxygen -is pumped out. Dynode 14 is made rpositive with respect to cathode Ztland is rotated vby :application of an external magnetic field to tab1'6 so that the formed-spot area of magnesium oxide coating 30 is directlyopposite cathode-2t), grid structure r18 beingintermediate'coating 3t and cathode 20. Cathode ZOis-energized byasuitable (notshown) potential source so that electrons-are emitted therefrom and, inasmuch as dynode 14 is positive with respect to cathode '20, the electrons bombard magnesium oxide coatingfit) setting up a secondary emission of several milliamperes. -At this point, the @grid structure is made positive with respect to the cathode, the secondary cmisison becomes self sustained and the magnesium =oxide coating emits electrons-even when the primary electron source, the cathodeiscut off. The selfsustainedemission is drawn from the coating to the grid structure :for several minutes to :removeany-gas thatrnayhave'been occluded in the oxide coating. Finally, the getter (not shown) is flashed and :the tube is sealed at a pressure of about 10* mm. of mercury, -all the ,potentials are removed from the various electrodes of the tube, and the :tube is now ready for operation.

The magnesium oxide film as produced above provides extremely large secondary emission ratios when bornbarded with electrons. It is to be noted that the magnesium oxide deposit has to be quite porous to obtain such high currents and this porosity is obtained by the evaporation of magnesium through oxygen, Contrariwise, a smooth deposit results when the evaporation is first carried out in a vacuum rather than through oxygen and then oxidizing the evaporated surface. When examined microscopically, the surface formed by evaporation through oxygen consists of a uniform distribution of small stalactite structures approximately 6Xl0- cm. long and 2X10 Cm. wide. In the case of the deposit carried out in vacuum, the shape of the grains after oxidation is generally more spherical and not of uniform size.

The high oxygen pressure used (20-40 mm. of mercury) prevents the magnesium from evaporating at low temperatures. The positive ion bombardment heats the magnesium sutficiently so that it becomes ignited and burns off forming magnesium oxide fumes which deposit as a spot of thin white film on the dynode. It is necessary to have an oxygen pressure in the range of 20-40 mm. of mercury as evaporation of the magnesium in lower pressures of oxygen results in both a very poor positive ion production at the initiation of the process and a low concentration of oxygen in magnesium oxide formation. This, in turn requires heating of the magnesium indirectly by a filament and evaporation of the magnesium at low temperatures and also results in incomplete oxidation. Consequently, where lower than critical pressures of oxygen are maintained, the magnesium oxide formed on the anode has to be further oxidized by R. F. heating and even when the latter step is done, the magnesium oxide film is not completely oxidized. The layers that are obtained from the oxygen pressure, accordingly, are also somewhat unstable and diflicult to reproduce. In the method of the present invention, the oxidation of the magnesium is complete due to high pressure of oxygen utilized and the, consequent higher temperatures and initial positive ion production. Therefore, the magnesium oxide layers are more exactly reproducible.

In understanding the mechanism of secondary emission from thin dielectric films such as magnesium oxide, it is to be realized that high D. C. fields are applied across these films while the film is being bombarded with primary electrons. The secondary emission ratio is found to increase exponentially with the field over a wide range of bombardment energy. It is believed that primary electrons bombard the uncharged magnesium oxide film thus liberating electrons by ordinary secondary emission with a ratio greater than unity. As a result of its high resistivity, the secondary emission surface acquires a large positive charge, and a high field is created across the film. Due to the porosity of the film, subsequent bombarding electrons will penetrate a fixed distance into the magnesium oxide. Secondary electrons released within the material are accelerated towards the surface of the film under the influence of this high field. The secondary electrons travel largely through the pores, gaining sufficient energy from the field to create further electrons by impact. Each new secondary electron creates additional electrons by internal ionization, and

a Townsend type avalanche results. As, hereinbefore described, the emission becomes self sustained and the limit of gain is finally reached when the surface of the magnesium oxide film approaches that of the collector grid, viz; the grid structure 18. If the surface potential becomes higher than that of the collector grid, the electrons return to surface, neutralizing some of the positive charges. If the surface falls below the grid potential, the emission of electrons will quickly charge the surface more positively and an equilibrium potential will be established. The surface of the film thus becomes a unipotential region of positive charges, similar to the condition existing in the cathode glow region of a gaseQlls glow discharge,

As set forth above, the magnesium oxide layers produced by the method of the present invention provides self sustained emission up, to 50 ma. which are stable and substantially reproducible. It is believed that the electron avalanche in the oxide is probably due to the fact that the oxide layers under microscopic examination are made up of stalactite like structures and are porous. The porous layers provide long electronic mean free paths, permitting the electrons to gain sutficient energy from the field between collisions to cause further ionization in the oxide, resulting in an electron avalanche.

Inasmuch as the porous magnesium oxide layer produces such extremely high secondary emission currents, it has been found that these secondary electrons have high initial velocities and may be utilized to provide vacuum tubes which can effectively be used as a noise generator.

The tube of Fig. 1 may be used as such a generator. In such a tube, the self sustained emission of magnesium oxide coating 30 on dynode 14 has run self sustained in excess'of 500 hours. By the use of rotatable dynode 14, the cleanness of grid structure 18 is assured in the fabrication of the tube. Of course, dynode 14 could be a fixed structure and grid structure 18 movable and in this manner also insure prevention of contamination on the grid.

In Figs. 2 and 3, there is shown a miniature tube noise generator wherein the present magnesium oxide coating is utilized. In this embodiment, the magnesium oxide is deposited in the manner as described in the tube of Fig. 1 from the magnesium receptacle 12 through grid structure 18 onto dynode 14. In this situation, grid structure 18 also becomes coated with magnesium oxide so that it is possible to obtain large, random, self sustained emission currents from grid structure 18 to dynode 14 by bombarding the grid with primary electrons and making dynode 14 positive with respect to the grid structure a suitable value being about volts. In any event, using either dynode 14 or grid 18 as the secondary emitter, it is possible to obtain self sustained emission up to about 50 ma.

In Figs. 4 and 5 there is shown, a subminiature tube noise generator utilizing the coating of the present invention. In this embodiment a cylindrical grid structure 32 is positioned concentrically within and spaced from cylindrical dynode 14. Filament 20 is positioned within grid structure 32 and magnesium receptacle 12 containing magnesium filings is situated above the dynode 12, grid 32 and filament 20 substantially along the axis of dynode 14. In the fabrication of this embodiment, the glow discharge is initiated between receptacle 12 and dynode 14 and grid 32 resulting in a magnesium oxide coating on the grid and the inner surface of dynode 14. To prevent any magnesium oxide from depositing on the filament, a disc 34 of nickel or the like metal is spot welded or atfixed in a suitable manner to the top of filament 20. As an added precaution, to prevent contamination of the layer by getter (not shown) metal such as barium when the getter is flashed, a mica shield (not shown) may be inserted between the getter and tube elements thereby shielding the magnesium oxide layer.

In the operation of these noise generator tubes, the dynode and grid are placed at the same potential, about 100 volts more positive than the filament, and the oxide layer is bombarded by primary electrons from the filament. The grid potential is then raised and the secondmy current from the oxide begins to exhibit a blue fluorescence which becomes more intense as the current increases. In effect, such a tube may be utilized to provide a visible display. At a secondary current of several hundred microamperes, the emission becomes self sus' tained. The self sustained current may then be varied by changing the grid potential and will show exponential dependence upon the difference of potential between the dynode and the grid.

While there have been described what at present are considered to be preferred embodiments of the invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore aimed in the appended claims to cover all such modifications as fall within the spirit and scope of the invention.

What is claimed is:

1. A method of preparing a magnesium oxide coating for an electrode in a discharge device, said coating being characterized by greatly enhanced secondary emissionwhen bombarded with electrons comprising filling with oxygen at a pressure of 20-40 mm. of mercury, an envelope having therein a metal receptacle containing magnesium and an electrode spaced from said receptacle, initiating the positive ionization of said oxygen, positively biasing said electrode with respect to said receptacle whereby a glow discharge is set up therebetween, said positive ions being attracted to said receptacle thereby igniting said magnesium, permitting said ignited magnesium to burn in said oxygen until said magnesium is completely oxidized whereby a thin, porous, stalactite like coating of magnesium oxide is deposited on said electrode.

2. A method of fabricating a discharge device comprising an envelope containing therewithin at least a cathode, an anode having a porous stalactite like magnesium oxide coating thereon, and a metal receptacle containing magnesium comprising introducing oxygen into said envelope at a pressure of 20-40 mm. of mercury, initiating the positive ionization of said oxygen, providing a glow discharge between said metal receptacle and said anode by biasing said anode positive with respect to said receptacle whereby said positive oxygen ions are attracted to said receptacle igniting said magnesium, permitting said magnesium to burn in said oxygen until said magnesium is completely oxidized whereby a porous stalactite like magnesium oxide coating forms on said anode.

3. A method as in claim 2 wherein said anode is made from 400 to 600 volts positive with respect to said receptacle.

4. A method of fabricating a discharge device comprising an envelope containing therewithin a cylindrical rotatable anode having a porous stalactite like magnesium oxide coating thereon, an emitting cathode spaced from a first portion of the cylindrical surface of said anode, an open mesh grid structure intermediate said cathode and said anode and a metal receptacle containing magnesium spaced from a second portion of the cylindrical surface of said anode comprising introducing oxygen at a pressure of 20-40 mm. of mercury into said envelope, initiating the ionization of said oxygen, providing a glow discharge by positively biasing said anode with respect to said receptacle whereby said ogygen ions are attracted to said receptacle and ignite said magnesium, permitting said magnesium to burn in said oxygen until said magnesium is completely oxidized whereby a porous stalactite like coating of magnesium oxide forms on said anode, exhausting said oxygen from said envelope.

5. A method as in claim 4 wherein said anode is biased from 400-600 volts positive with respect to said metal receptacle.

6. A method of fabricating a discharge device which provides self sustained secondary emission comprising an envelope containing therewithin an anode, having a porous stalactite like magnesium oxide coating thereon, an emitting cathode opposite a first portion of the anode surface, a metal receptacle opposite a second portion of the said anode surface and a grid intermediate said cathode and receptacle and said anode comprising introducing oxygen at a pressure of from 20-40 mm. of mercury into said envelope, initiating the positive ionization of said oxygen, providing a glow discharge between said anode and grid and said receptacle by biasing said anode and grid from 400-600 volts positive with respect to said receptacle whereby said oxygen ions are attracted to said receptacle and ignite said magnesium, permitting said magnesium to burn in said oxygen until said magnesium is I completely oxidized whereby said porous stalactite like 1 magnesium oxide coating forms on said grid and said anode.

7. A method of preparing a discharge device comprising an envelope containing therewithin a substantially cylindrical anode having a porous stalactite like secondary electron emitting magnesium oxide coating thereon, a substantially cylindrical grid concentrically positioned within and spaced from said anode, an emitting cathode positioned within said grid, and a metal receptacle containing magnesium positioned at point outside of and substantially along the axes of said cylinders comprising introducing oxygen at a pressure of from 20-40 mm. of mercury into said envelope, initiating the positive ionization of said'oxygen, providing a glow discharge between said anode and grid and said receptacle by biasing said anode and grid positive with respect to said receptacle whereby said oxygen ions are attracted to said receptacle and ignite said magnesium, permitting said magnesium to burn in said oxygen until said magnesium is completely oxidized whereby a porous stalactite magnesium oxide coating forms on said grid and said anode.

8. A discharge device comprising an evacuated envelope including therewithin at least an emitting cathode v and an anode having a porous magnesium oxide coating thereon, said coating consisting of magnesium oxide crystals of stalactite like structure and having a high secondary to primary emission ratio.

9. A discharge device characterized by high self sustained secondary emission comprising an evacuated envelope including therewithin an emitting cathode, an anode having a porous magnesium oxide coating thereon, said coating consisting of magnesium oxide crystals ofstalactite like structure, and a grid intermediate said anode and cathode.

10. A noise generator comprising an evacuated envelope including therewithin a primary electron emitting cathode, an anode having a secondary electron emitting porous magnesium oxide coating thereon, said coating consisting of magnesium oxide crystals of stalactite like structure and having a high secondary to primary emis sion ratio, a grid intermediate said anode and cathode, means for energizing said cathode, means for biasing said anode positive with respect to said cathode whereby a portion of said secondary electron emission from said anode is available for noise.

11. A visible display tube comprising an evacuated envelope including therewithin at least a primary electron emitting cathode, an anode having a secondary electron emitting porous magnesium oxide coating thereon, said coating consisting of magnesium oxide crystals of stalactite like structure and having a high secondary to primary electron emission ratio, the secondary electron current from said coating providing a fluorescent display.

References Cited in the file of this patent UNITED STATES PATENTS 1,802,474 Lebrun et a1 Apr. 28, 1931 2,146,607 Van Overbeek Feb. 7, 1939 2,151,783 Lopp et a1 Mar. 28, 1939 2,171,227 Schreinemachers Aug. 29, 1939 2,198,329 Bruining et a1. Apr. 23, 1940 2,401,040 Becker May 28. 1946 2,526,574 Mendenhall Oct. 17, 1950 

8. A DISCHARGE COMPRISING AN EVACUTED ENVELOPE INCLUDING THEREWITHIN AT LEAST AN EMITTING CATHODE AND AN ANODE HAVING A POROUS MAGNESIUM OXIDE COATING THEREON, SAID COATING OF MAGNESIUM OXIDE CRYSTALS OF STALACTITE LIKE STRUCTURE AND HAVING A HIGH SECONDARY TO PRIMARY EMISSION RATIO. 